O2, Three, Vodafone and EE customers will be hit with price hikes later this year. The news comes as all the major mobile phone networks announce mid-contract increases.

EE is to increase the price by 4.1%, for millions of its users - the changes will come into effect from 30 March. The network provider is currently contacting customers about the rise.

The changes will affect all pay-monthly mobile customers unless they joined EE or upgraded after February, 6 2018. Pay-as-you-go customers won't see prices go up.

Meanwhile, Vodaphone and O2 have said their customers can expect a payment increase in April.

Three is the only network provider yet to confirm when its rise will take effect but in 2017 it happened in May - although this is no indication as to this year's date.

Why are the prices going up?

The price increases are based on the Retail Price Index (RPI). This is a measure of inflation published each month by the Office for National Statistics.

Inflation rates give a snapshot of how much prices have risen by over a 12-month period.

Using this information, mobile phone networks - as well as other big companies - are allowed to enforce mid-contract price hikes.

Can I leave my network mid-contract and avoid a penalty cost?

Mobile networks can increase prices mid-contract in line with inflation, as long as they include a warning and information about it in their terms and conditions.

According to moneysavingexpert.com, Three and O2 do, and Vodafone does for contracts taken out on or after 5 May 2016.

Consumer rights site which.co.uk says: "Rules set by the regulator Ofcom mean that customers can leave mobile, landline or broadband contracts penalty-free if a provider ups prices mid-term - but only if it didn’t warn you about the rise.

"You may also be allowed to leave without paying a charge if the rise is of 'material detriment', for example a rise that's bigger than the RPI rate.

"If your mobile phone provider has warned you about rises in their terms and conditions and they are in line with RPI, you won’t be able to leave if you’re still locked into a contract.

"If you do want to leave, you'll have to pay an exit fee."

What if I'm already out of my contract?

If you're already out of your minimum contract term then you can leave without paying a penalty fee, and switch to another deal.

What are the mobile network providers saying?

Speaking to moneysavingexpert.com, an EE spokesperson said: "Like many service providers, our pay-monthly plans increase by RPI annually, and this year customers on our most popular plan will typically see an increase of 85p a month."

A Vodafone spokesperson said: "All new pay-monthly contracts taken out on or after 5 May 2016 have an annual price adjustment in line with the RPI. For consumer contracts, we use the RPI figure published by the Office for National Statistics in March and apply the change in April each year, from 2017."

An O2 spokesperson said: "We know that our customers want transparency and control over their monthly bills and, importantly, don't want to be forced to pay for a phone they already own. By splitting the airtime and device plan, we've helped our O2 Refresh customers avoid paying millions of pounds."

Meanwhile, a Three spokesperson said that the network provider does not intend to impose an increase equivalent to RPI this year.

She said: "This year, our customers will see an increase in line with our terms and conditions.

"We know price increases are frustrating but we are committed to giving customers the best experience possible by offering benefits such as roaming in 71 worldwide destinations at no extra cost, streaming the latest TV shows and music without eating into data, 4G at no extra cost, plus access to weekly rewards and free stuff through our Wuntu app."